Intrahepatic activation of naive CD4+ T cells by liver-resident phagocytic cells.

Naive T cell activation is normally restricted to the lymphoid organs, in part because of their limited ability to migrate into the parenchyma of peripheral tissues. The liver vasculature is unique, however, and circulating leukocytes within the hepatic sinusoids have direct access to liver-resident cells, which include an abundant population of Kupffer cells. It is well accepted that recognition of cognate Ag within the liver leads to naive CD8(+) T cell activation in situ, but it is unclear whether the liver also supports naive CD4(+) T cell activation. In this study, we show that naive CD4(+) T cells can be activated to proliferate in the liver when cognate Ag expression is induced in hepatocytes by recombinant adeno-associated viral vectors. Ag-specific retention and activation of naive CD4(+) T cells within the liver are independent of lymphoid tissues but dependent on a clodronate liposome-sensitive population of liver-resident phagocytic cells. To our knowledge, this study provides the first unequivocal evidence that naive CD4(+) T cells can be activated in a nonlymphoid organ. It also gives critical insight into how CD4(+) T cells specific for Ag expressed in the liver are recruited to participate in protective or pathological responses during hepatotropic infections and autoimmune liver disease.

Differential migration of passenger leukocytes and rapid deletion of naive alloreactive CD8 T cells after mouse liver transplantation.

Donor passenger leukocytes (PLs) from transplanted livers migrate to recipient lymphoid tissues, where they are thought to induce the deletion of donor-specific T cells and tolerance. Difficulties in tracking alloreactive T cells and PLs in rats and in performing this complex surgery in mice have limited progress in identifying the contribution of PL subsets and sites and the kinetics of T cell deletion. Here we developed a mouse liver transplant model in which PLs, recipient cells, and a reporter population of transgenic CD8 T cells specific for the graft could be easily distinguished and quantified in allografts and recipient organs by flow cytometry. All PL subsets circulated rapidly via the blood as soon as 1.5 hours after transplantation. By 24 hours, PLs were distributed differently in the lymph nodes and spleen, whereas donor natural killer and natural killer T cells remained in the liver and blood. Reporter T cells were activated in both liver and lymphoid tissues, but their numbers dramatically decreased within the first 48 hours. These results provide the first unequivocal demonstration of the differential recirculation of liver PL subsets after transplantation, and show that alloreactive CD8 T cells are deleted more rapidly than initially reported. This model will be useful for dissecting early events leading to the spontaneous acceptance of liver transplants.